The commercial advantage of the fluid-bed reactor is the ability of a fluidized-solid bed to approach isothermal conditions. This makes it possible to control the temperature closely, which in turn permits close control of the process. Though there are fluid-bed processes in which no reaction takes place, this invention is concerned only with commercial processes where a chemical reaction occurs at elevated temperature and pressure, which processes we have found, are unexpectedly correlatable in operation with a laboratory fluid-bed process operating at ambient pressure. More particularly, this invention is related to those processes using chemical reactions in fluid-bed catalysts which, we have also found, display a peculiar phenomenon best described as "tackiness" or "stickiness" which is not necessarily viscosity, as conventionally defined. Stickiness maybe defined as the degree of particle-to-particle agglomeration, viscosity or resistance to movement or separation of constituent particles. Stickiness of a support catalyst is dependent upon the pressure and temperature of the catalytic reaction, the adsorptive quality of the catalyst, the amount and distribution of active ingredient on the surface, the number of active sites available on the catalyst and the manner and degree of their utilization, and the quantity and physico-chemical properties of the reactants and reaction products in the fluid-bed.
It will be appreciated that fluid-bed catalysts, such as are used in commercial reactors in chemical processes, consist essentially of an attrition-resistant, usually porous catalyst support on which is deposited one or more essential catalytic ingredients. A typical catalyst support is silica, kieselguhr, fuller's earth, pumice, and alumina, the last named being generally preferred because of its superior resistance to attrition, its ability to fluidize and especially because it can be prepared with a desired surface area, and in a preselected ratio of particle sizes. Except for a change in color and surface area, depositing a desirable catalytic ingredient on the catalyst support does not substantially change the physical characteristics of the catalyst support, which closely resembles fine sand. In the normal course of a conventional start-up of a fluid-bed reactor, the fluid-bed catalyst gives no indication that it might undergo a change in "consistency", however this may be defined. But at elevated temperatures above about 100.degree. C. but below a temperature deleterious to the catalyst, particularly while the catalyst is catalyzing a reaction between feed components fed to the reactor, primary particles of some catalysts exhibit a proclivity to stick to one and another, thus displaying an overall change of fluid-bed consistency within the reactor. Catalysts which exhibit such a tendency include catalyst supports on which are deposited the "soft" elements of Groups I, V, VI and VIII of the Periodic Table, and compounds thereof. Most susceptible to a change in consistency are supported catalysts on which are deposited compounds of copper, iron, bismuth, antimony and the like, and which additionally may be promoted by the rare earth elements and elements of Groups II, IV, and VII. Numerous chemical processes utilizing supported catalysts in a fluid-bed reaction are listed in the chapter entitled "Fluidization" in Encyclopedia of Chemical Technology, Kirk-Othmer, Vol. 9, p 400-404, 2d Edition, Interscience Publishers, John Wiley & Sons, Inc. (1966). Many of these catalysts are stickiness-prone during operation of the fluid-bed reactor containing the catalyst.
In specific processes listed in Kirk-Othmer (supra), such as the ammoxidation of propylene to make acrylonitrile, the oxidation of propylene to make acrolein, the oxyhydrochlorination of lower alkanes and alkanes having from 1 to 4 carbon atoms, and particularly methane to make carbon tetrachloride, chloroform, etc., and the oxyhydrochlorination of ethylene to make 1,2-dichloroethane (referred to as "EDC"), close control of these reactors is conventionally exercized by monitoring flow rates and temperatures of feed components, the pressure drop and temperature profile in the fluid-bed, reactor effluent analyses, heat duty of condensers, and the like. To obtain a good indication of what is widely believed to be the precise condition of an operating fluid bed, it is conventional to monitor the bed height, corresponding bed density and pressure drop continuously across the bed, and to record an hourly (or half-hourly) moving average bed height, bed density and pressure drop. With due experience, by correlating bed height and pressure drop with efficiency, one may continually estimate efficiency of the overall reaction. By "efficiency" we refer to conversion of one or more feed components to desirable products at mimimum cost. Desired operating conditions for a reactor (referred to in the plant as "normal optimum" operating conditions) are those which provide maximum conversion of one or more feed components to desirable products at mimimum cost. However, despite frequent pressure drop and bed height measurements derived from just-elapsed conditions, the estimate of efficiency is relatively long term, and of no value to predict an impending process "upset", whatever its cause. There is no direct correlation between bed height and the inversion point for the bed. Moreover, especially in a large operating fluid-bed reactor, any warning related to a process upset attributable to a dangerous change in bed height or a change in pressure drop is usually measured much too late to correct the conditions which gave rise to the process upset.
The importance of time will be more readily recognized when it is realized that a large fluid-bed reactor, operating at desired operating conditions has little latitude in acceptable process conditions. Moreover, in an operating fluid-bed having a quantum of stickiness, typical for a particular supported catalyst, we have found that a process upset can change the stickiness quite suddenly, long before an alarming change in pressure drop is measured. When the change in pressure drop is registered, even drastic changes in process conditions fail to restore the fluid-bed to its pre-upset operating stability. Very soon there is no alternative but to shut the reactor down. Stated differently, we have found that a process upset which causes a sudden change in stickiness of a fluid-bed operating at desired conditions, presages a process "point-of-no-return" or the "inversion point" for the fluid-bed, without immediately and significantly altering the pressure drop across the reactor. If one waits for the alarm to be set of by a sudden rise in pressure drop, or sudden collapse of the bed, it is too late. The reactor is soon shut down.
It will now be evident that, in an operating commercial fluid-bed reactor using a supported catalyst having a proclivity to increase in stickiness as conversion of reactants to a desired product improves, the risks attendant operation of the reactor at peak efficiency, near the inversion point of the fluid-bed, are too great. As a result, such a reactor is operated well short of peak efficiency with concomitant higher costs for feed components and operation of the recovery and purification systems. Pressure drop across the fluid-bed, the height of the fluid-bed, and the composition of the effluent, may all be monitored to control overall efficiency. If the fluid-bed is upset by process conditions which place the fluid-bed beyond the inversion point, the reactor is soon shut down. Though it is self-evident that the inversion point of a fluid-bed may be determined by running the bed at conditions from which there is no recovery, there is no reliable way of estimating when that inversion point may be reached under different process conditions with a particular catalyst, or with a different catalyst at the same process conditions. Measurement of changes in pressure drop by itself, is not generally correlatable with changes in stickiness. In some processes, for example in the oxyhydrochlorination of ethylene to produce EDC, pressure drop is actually found to decrease across the bed while the stickiness increases. Of course, once the inversion point is exceeded, the pressure drop will increase until the bed solidifies. There is no reason to expect that a supported catalyst should exhibit a proclivity to increase in stickiness as conversion of reactants to a desired product improves, or that the inversion point of a fluid-bed is immediately preceded by a sudden rise in stickiness, but it is, and the process of this invention is based on this finding.
It has been suggested to measure a "pseudoviscosity" or stickiness which might be a fundamental correlant of slugging, good fluidity or other bed characteristics (see Matheson, G. C., Herbst, W. A. and Holt, P.H., Ind. Eng. Chem. 41, 1099 (1949)). It has not be suggested that by frequently monitoring the relative pseudo-viscosity, stickness, tackiness or consistency (all of which terms are hereafter referred to simply as "viscosity", for brevity) of an operation fluid-bed in which a chemical reaction takes place, the data might be used to maximize efficiency of the reaction; nor that a sudden rise in "viscosity", relative to the viscosity at usual operating conditions, might signal a danger-point requiring immediate attention. Neither has it been suggested that the viscosity characteristics of supported catalyst in a laboratory fluid-bed reactor operating at atmospheric pressure, may be correlateable with those of a commercial or plant fluid bed reactor operating at elevated pressure.
Recognizing that a fluid-bed displays "viscosity", a pendulum viscometer having damped torsional oscillation has been used to make measurements which are empirically correlatable to obtain a rate of damping (see "Viscometers Having Damped Torsional Oscillation", by Ashwin, B. S., Hagyard, T., et al J. Scientific Instruments, 37, p 480-485, December 1960). However, the reference is unconcerned with a reactor and does not suggest that flow rates are relatively insignificant in comparison with the reaction conditions, or change in particle size due to agglomeration induced by changes in reaction conditions. It is known that reliable rates of damping cannot be obtained with vibrating-reed type viscometers, or viscometers having quite small clearances, as in the capillary flow and the concentric cylinder type, all of which are unsuited for the purpose. Similarly, powered viscometers in which the power to maintain a predetermined rotation is measured, are unsuited in a fluid-bed reactor because there is too much disturbance of the particles in the immediate vicinity of the viscometer. It is essential that the rate of damping be measured reliably and reproducibly, and to our knowledge, this can only be done with a pendulum viscometer which uses relatively low rotational velocity, and which does not appreciably disturb the state of the fluid bed in the immediate vicinity of the viscometer.